1. Field of the Invention
The present invention is directed to an improved method and apparatus for regenerative thermal oxidation. More specifically, it is a method and a thermal oxidizer for purifying gases by incineration while providing regenerative heat exchange between inleting and outleting gases, and, optionally, providing for purging of gases in individual thermal chambers. Such systems are used in commercial and industrial operations for removal of air pollutants from gases which would otherwise be released to the atmosphere. By thermal oxidation, these gases are raised to their auto ignition temperature, creating safer, and more simple exhaust gas compounds. By using proper heat exchanger techniques, as described herein, the efficiency in the cost of operation is enhanced. A majority of applications in pollution control involve condensable organics in process exhaust. Conventional inlet butterfly valves or dampers become extremely susceptible to these condensables, which will cause improper valve sealing and lower destruction efficiencies. Many manufactures have devised systems to control the build-up of condensables on their inlet valves. All of these control systems can only be run when the system is not processing fume, and with added cost to the base system. The present invention does not have separate "inlet" or "outlet" valves, only one blade that continuously cycles between inlet and outlet modes. This continuous cycling will cause the blade to reach an equilibrium temperature between the inlet and outlet temperatures. This means the blade will, always be hotter than the inlet air stream and will not cause process fumes to condense. The specific improvement, and preferred embodiment, involves the utilization of hot exhaust air to pressurize the area around the gates in order to minimize leakage and to increase thermal and oxidation efficiency.
2. Information Disclosure Statement
Regenerative thermal oxidizers such as in the present invention, utilize a back flow of gases through the heat exchanger to maximize the recapture of heat which might otherwise be lost to the atmosphere. These systems include minimally a combustion chamber which receives the preheated gas, and recovery chamber which cools the gas exiting so as to reclaim most of the heat, to be applied to the inleting process gas before entering the combustion chamber. A burner is used to maintain temperature in the combustion chamber. Typically the system operates dynamically holding inlet and outlet conditions for a predetermined period of time before continuing on to the next inlet/outlet condition. In this manner, the regenerator could become one of a heating regenerator rather than an exhaust regenerator and the portion or unit which constituted the heating regenerator could become the cooling regenerator.
Early regenerator systems involved the use of, for example, two chambers wherein the gases to be purified entered into the bottom of one chamber, rose up therethrough for oxidation, and back down through and out a second chamber unit for capture of the heat. Operated in this manner, for a period of time, the process was reversed so that the inlet gas is now moved first into what was the exhaust chamber utilizing the energy for preheat. One of the problems which existed during the flow reversal involved large pressure swings which create back pressure effecting manufacturing systems which require equilibrium within the system. Oxidizers with three or more chambers can make use of an idle chamber which transitions with one or more of the other chambers to maintain an even flow across the systems during changes in airflow direction through the equipment. Difficulties arose with respect to the manifolding of these gases because, historically, flap valves which rotated were utilized to provide the inleting and outleting of the process exhaust gases. This caused inefficiencies, improper sealing problems, mechanical wear and tear due to the rotational aspects thermal expansion problems, and pockets of process air which would not be treated after the flap valves transitioned from inlet to outlet positions. In addition, flap valves require a complicated array of control to insure each valve is opening and closing properly. In fact prior art makes no provision for allowing both inlet and outlet flap valves from opening and closing simultaneously, allowing contaminated air to bypass the oxidation chamber and be exhausted, unprocessed, to the atmosphere.
Various solutions have been developed to address problems incurred in regenerative heat chambers, although the prior art does not teach or suggest the present invention described herein. The following patents are representative of the state of the art:
U.S. Pat. No. 3,741,286 issued to Wolf Muhlrad described a regenerative heat exchanger and method for purging its flow passages. The invention described involves double chambers with gases passing up one side and down the other and then reversing. This is accomplished by rotation of a series of dampers which do not eliminate the above mentioned problems with rotational valves.
U.S. Pat. No. 4,470,806 issued to Richard Greco described regenerative incinerators utilizing a series of heat exchanger chambers with perforated horizontal support grates with combustion chambers located above heat exchangers and utilizes openings and closing of various lines to achieve reversals for heat exchange. However, as in the other prior art, this patent utilizes valves which rotate to open and close the inlet and outlet manifolds.
U.S. Pat. No. 3,225,819 issued to E. S. Stevens describes an apparatus and method for air-to-air heat exchange, but again, utilizes rotational valving.
U.S. Pat. No. 4,754,806 describes a reciprocating heat exchanger and utilizes a porous metal element for absorbing heat for the exhaust air stream and also describes movement of this porous metal element from one side to another so as to effectively exchange heat by receiving heat from exiting gases and moving it over to inlet gas lines. However, this patent does not describe valving or manifolding and takes a totally different approach by never reversing the flow of gases but only reciprocally relocating heat absorbing elements.
U.S. Pat. No. 4,966,228 and U.S. Pat. No. 5,026,277, describe some recent developments of the art in this field. The latter patent to James York describes a regenerative thermal incinerator apparatus utilizing a system of valved duct work to direct gases to various combustion chambers and to idle a third regenerator for purging of partially treated gas so as to recycle it back through the system and so as to, thereby, reduce or eliminate lost gases which have not been properly purified.
U.S. Pat. No. 4,280,416 issued to Phillip Edgerton, describes a rotary valve for regenerative thermal reactors. While this patent describes valves which rotate instead of swing or rotate about an axis parallel to the valve surface, it still requires rotation of a valve about an axis at right angles to its surface and rotational friction and mechanical drive is necessitated.
U.S. Pat. No. 4,966,228 issued to Sherwood Fawcett describes a regenerative gas-to-gas heat exchanger requiring a special geometry for a chamber and utilizing gate valves to direct or, redirect gases through different chambers. Unlike the present invention, however, Fawcett does not reverse gas flow and Fawcett does not rely upon the use of sliding gates having an orifice which is moved from an inlet to dead space or closed off space to an outlet and recycled back and forth so as to control the flow of gases in one direction and then in the exact opposite direction within a specific chamber unit.
U.S. Pat. No. 5,134,945 to the present inventors herein describes the present state of the art in the field. The thermal oxidizers described therein utilize at least three regenerative thermal chamber units and these have on a single surface both inlet and outlet openings with slidable, controlled and timed gates to set opened or closed each of the inlet and outlet openings to shift regeneration inlet and outlet from chamber unit to chamber unit. This prior art patent does disclose overall oxidizer purging based on inleting purge gases through the top of the entire set of units for purging, but neither suggests nor teaches the pressurization of the units by sealing of the gate manifolds, as set forth in the present invention.